A flat-bed image scanner is disclosed in e.g. Patent Document 1. The image scanner utilizes an image sensor unit U in which a CCD line sensor is mounted. (Hereinafter, this unit is referred to as a “CCD image sensor unit”). As shown in FIG. 11 of the present application, such a CCD image sensor unit U generally includes an illumination light source 1, a plurality of mirrors 21-25, a lens 3 and a CCD line sensor 4 which are housed in a case 5. In the flat-bed image scanner S, the CCD image sensor unit U is moved in the secondary scanning direction (right and left direction in FIG. 11) below a document table DP made of e.g. transparent glass. In the operation to read the image of the document, the light emitted from the illumination light source 1 and then reflected by the document D is reflected by the mirrors 21-25 to converge on the CCD line sensor 4 via the lens 3. Thus, the image of one line of the document D extending in the primary scanning direction (the direction perpendicular to the sheet surface in FIG. 11) is formed on the CCD line sensor 4 and read. This operation is repeated every time the CCD image sensor unit U is moved in the secondary scanning direction by a predetermined pitch, whereby the two dimensional image of the document is read.
In the flat-bed image scanner S utilizing the CCD image sensor unit U, the light path from the document D to the CCD line sensor 4 is relatively long, so that a large depth of field of the lens 3 is provided. Thus, even when the document D is slightly separated from the surface of the document table DP, clear image is obtained without defocusing.    Patent Document 1: JP-A-2000-134413
Generally, a cold-cathode tube which emits white light is used as the illumination light source 1 of the CCD image sensor unit U. This is because, at present, in view of the cost and so on, a cold-cathode tube is considered to be the optimum linear light source device for irradiating the document with light of uniform illuminance throughout the entire reading range extending in the primary scanning direction for reading a color image.
However, the use of a cold-cathode tube as the linear light source device of a CCD image sensor unit U has drawbacks as follows.
Firstly, to drive a cold-cathode tube, a high voltage for discharge generated by using e.g. an inverter is necessary, so that the cost for the power supply circuit is high.
Secondly, a cold-cathode tube is not good for environment, because mercury vapor, which is harmful, is enclosed.
Thirdly, although a cold-cathode tube emits uniform light throughout the length, the amount of light received by the light receiving elements of the CCD line sensor 4 becomes smaller as progressing toward each end. Specifically, in the CCD image sensor unit U, the reading range of e.g. A4 size is finally reduced to the width of the CCD line sensor 4 which is about several tens of mm. Thus, the angle of view as viewed from the CCD line sensor 4 or the lens 3 is about 50° (see FIG. 12). Thus, the receiving amount of light reflected by the document becomes smaller as progressing toward each end of the reading range. Although such a reduction in the light receiving amount can be corrected, such correction requires a complicated correction circuit particularly when a color image is to be read.
Fourthly, the light emitted from a cold-cathode tube is not used efficiently. Specifically, although illumination is performed only in one direction, a cold-cathode tube emits light from the entire circumference, so that much light is wasted. To solve this problem, a reflective member needs to be arranged behind the cold-cathode tube, which increases the cost.